I've ruled this out as a possibility. For instance here I was the 4th comment (there are now 16), I posted 28 minutes ago and I'm still at the top comment slot at only 1 point.
It must be more than just a function of time and score, otherwise any of the new 1-point comments would have superseded mine, but they didn't.
Your average karma is 24, so your comments are weighted much, much more than others (it's one of metrics HN uses to rank comments). Also, your comment is long, which is also a factor (though, not a big one).
If you take comment length + karma per post / comments on post you should end up with reasonably decent comments making it to the top. Ish. It's still never going to be able to fully deal with all the corner cases on a site as large as HN but it's often Good Enough.
It would make sense if your comments were weighted higher having a higher karma, but I think all of our comments start at the top to make sure that at least someone sees them and that they can get upvotes if they are insightful/interesting.
It must be more than just a function of time and score, otherwise any of the new 1-point comments would have superseded mine, but they didn't.